1. Field of the Invention
The present invention relates to a rubber composition and to a pneumatic tire formed using the rubber composition and, specifically, to a rubber composition that is excellent for use in a tire tread and that prevents deterioration in wet skid performance, and to a pneumatic tire formed by using this rubber composition that has excellent braking and control performance on icy and snowy road surface.
2. Description of the Related Art
Tires that run on normal road surfaces are formed using a rubber composition that mainly comprises a styrene-butadiene copolymer that shows a comparatively high glass transition temperature as the composition for the tire tread. This is because importance is given to the braking performance, as well as the ability to control the tire, of the tire on a wet road surface (wet skid performance). In contrast, tires for use in a winter are formed using mainly polybutadiene rubber or natural rubber that shows a low glass transition temperature because importance is given to the braking performance of the tire on ice or snow (ice skid performance).
Ice skid performance places importance on increasing the coefficient of friction with the road surface by increasing the level of flexibility in the tread rubber in low temperatures (e.g. approximately xe2x88x9220xc2x0 C.). In low temperatures, the modulus of elasticity of the rubber composition is raised inhibiting the tire from following the bumps and indentations in the road surface, and on the other hand, an icy road surface has fewer bumps and indentations than a normal road surface. As a result, the contribution of the energy dissipation (tanxcex4) generated between the rubber and the road surface to the ice skid performance is reduced. In low temperature conditions, it is necessary to increase the size of the real contact surface area between the rubber and the road surface, so that is more vital to lower the storage modulus of elasticity Exe2x80x2 (elastic modulus reduction) in the area of xe2x88x9220xc2x0 C. If the modulus of elasticity is lowered in low temperatures when a normal BR is used, however there is a remarkable deterioration in the wet skid performance. Therefore, attempts have been made to suppress this deterioration in the wet skid performance by using silica (SiO2) as a filler in a tread rubber composition. Although the wet skid performance can be improved by using silica as a filler in this way, the performance is still not satisfactory compared with a tire designed for use in normal conditions (i.e. non winter conditions).
Moreover, if SiO2 is blended in large quantities, the electric resistance of the tread rubber increases, therefore, static electricity is easily generated from the tire surface throughout the vehicle, leading to radio noise being generated, as well as giving rise to a fear of danger from sparks being giving off while the car is being filled with gasoline, and the like.
The present invention was conceived in view of the above problems. It is an object of the present invention to provide a rubber composition and a pneumatic tire formed using that rubber composition as its tread rubber, the rubber composition that has excellent driving control and braking performance on icy or snowy road surfaces, namely, ice skid performance, and is able to provide satisfactory wet skid performance by preventing any reduction in wet skid performance.
As a result of intense studies, the present inventors have noticed that the above problems can be solved by using a low molecular weight cis form butadiene rubber, and thus achieved the present invention.
Namely, the rubber composition of the present invention comprises a 100 parts by weight of rubber component comprising. (1) 5 percent by weight to 50 percent by weight of low molecular weight butadiene rubber (BR) having a cis 1,4-structure of 60 mol % to 98 mol % and a weight average molecular weight (Mw) of 5000 to 80000; and (2) 50 percent by weight to 90 percent by weight of styrene-butadiene rubber (SBR) having a bound styrene content of 15 percent by weight to 45 percent by weight and amount of a vinyl bonding in the butadiene portion of 7 mol % to 60 mol %; and 40 parts by weight to 95 parts by weight of silica. In addition, the pneumatic tire of the present invention is provided at least with a tread portion, and the rubber composition that forms at least the portion of the tread portion that makes contact with the road surface is formed using the above rubber composition.